Combine sourdough discard, milk, egg, and vanilla with an immersion blender until no streaks or lumps of sourdough starter or egg remain.
Add dry ingredients (flour, sugar, baking powder, salt) to wet ingredients and blend well until combined. Cover and rest 30 minutes.
Heat about 3" of neutral oil to 350°F in a wide, high-sided skillet with a candy thermometer attached.
Arrange a cooling rack over paper towels in a sheet pan and set next to the stove.
Insert a wire spider into the oil so that the rim of the spider breaks the surface of the oil.
Plug the bottom of the funnel with your finger and fill the funnel with about ¼ cup batter. Drizzle batter into the center of the wire spider in circular squiggle patterns using the funnel. Fry for 1 minute, then flip with the wire spider and fry an additional 30 seconds, or until golden brown on both sides.
Remove funnel cakes to cooling rack. Let rest 10-15 seconds, then dust with powdered sugar. Serve immediately.
For a more intense sourdough flavor cut the amount of sugar in half.
Don't have an immersion blender? A whisk works just as well.